A wire electric discharge machining device is a device that machines a workpiece using arc discharge generated between electrodes constituted by a wire electrode and the workpiece that are arranged to face each other. This wire electric machining device improves surface roughness by lowering a machining current from a rough machining condition using a relatively high machining current (such as a current at a pulse width of about several tens of microseconds) step by step, and by using a finish machining condition using a machining current at a current pulse width finally reduced down to about several tens of nanoseconds. To achieve this, some wire electric machining devices include a plurality of switchable power supply devices (power supply devices for an electrical discharge machine) that can supply a machining current corresponding to various machining conditions from the rough machining condition to the finish machining condition, respectively.
In finish machining, a high-frequency voltage in the order of megahertz is generally used as a machining voltage so as to improve the surface roughness of the workpiece, and a sinusoidal voltage that can be realized with a simple configuration is used as the high-frequency voltage or the machining voltage.
There have been conventionally proposed various types of power supply devices for an electrical discharge machine for improving the surface roughness of a workpiece (see, for example, Patent Documents 1 to 3).
A power supply device for an electrical discharge machine disclosed in Patent Document 1 is described using reference numerals shown in FIG. 1 of the patent document. The power supply device includes a direct-current (DC) power supply V1, a switching element S1 having one end connected to a positive electrode terminal of the DC power supply V1 via a line inductance LINE (a reactor) and to one end of an inter-electrode GAP, and a switching element S2 having one end, along with the other end of the switching element S1, connected to a negative electrode terminal of the DC power supply V1 and having the other end connected to the other end of the inter-electrode GAP.
If the power supply device is configured as described above, a boost-up voltage is generated when the switching element S1 performs a switching operation. At this time, the boost-up voltage charges a floating capacitance Cf present in the switching element S2 and the inter-electrode GAP by keeping the switching element S2 to be turned off. The surface roughness can be expected to improve by a surge short-pulse current flowing in the inter-electrode GAP at the time of charging. Further, the floating capacitance Cf limits the current flowing in the inter-electrode GAP. After a flow of a predetermined amount of current in the inter-electrode GAP, the switching element S2 is turned on. Electric charge accumulated in the floating capacitance Cf is thereby consumed, and the switching element S2 returns to an initial state. A capacitor can be provided in parallel with the switching element S2.
Patent Document 2 discloses a power supply device for an electrical discharge machine, which includes a high-frequency power supply having a resonance frequency determined by a floating capacitance and an inductance between electrodes. Because of use of resonance, the voltage generated between the electrodes is not a bipolar-pulse voltage but a sinusoidal voltage.
Moreover, Patent Document 3 discloses a technique capable of obtaining high surface roughness by changing levels of a machining current using a positive pulse and a negative pulse. It is considered that the high surface roughness can be obtained due to a change of an inter-electrode discharge state between a time of applying the positive pulse and a time of applying the negative pulse.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-329498    Patent Document 2: Japanese Patent Application Laid-open No. H5-177435    Patent Document 3: Japanese Patent No. 3361057